The present invention relates to a two-fluid pressure nozzle of upward injection type, a spray dryer using the nozzle, a spray dryer employing a closed system wherein the waste gas after spray drying is used as a hot gas for drying and/or a high-speed gas for atomization, and a method for control of droplet diameter in the nozzle.
Generally in spray dryers, a nozzle is provided at the top of a drying chamber with the nozzle tip directed downward; a feed liquid is blown downward from the nozzle to spray the liquid; the spray is contacted with a hot gas fed from the top of the drying chamber to effect drying and solidification. The downward blowoff of feed liquid is based on a conventional thinking that the downward blowoff utilizing gravity gives spraying of lesser turbulence and is more effective.
Further in spray dryers, there is generally employed an open system wherein the hot gas for drying and the high-speed gas for atomization are always fed as a fresh gas.
Spray dryers have conventionally used a rotary disc, a two-fluid nozzle or a centrifugal pressure nozzle.
The rotary disc makes easy change of particle diameter, switch between water and feed liquid, and change of liquid flow rate, but requires a high-speed rotating machinery and makes the maintenance complex. The two-fluid nozzle sprays a feed liquid by means of a high-speed air stream, but requires a large amount of high pressure air and incurs a high operating cost. Hence, the centrifugal pressure nozzle is preferentially used generally.
With the centrifugal pressure nozzle, a pressurized liquid receives a centrifugal force and thereby is sprayed from the orifice in a thin film of hollow cone shape; the hollow cone-shaped film is broken at the front periphery by the interaction with air; thereby, fine droplets are formed. In the centrifugal pressure nozzle, the diameters of droplets formed by spraying are controlled by the pressure applied to the feed liquid; however, change in pressure invites change in liquid flow rate. Therefore, control of droplet diameter is effected while the liquid flow rate is kept constant by using nozzle parts of appropriate combination and adjusting the spraying pressure.
In obtaining a powder product having a large average particle diameter or a dried product of a substance having a large specific gravity, for example, in obtaining large ceramic particles having an average particle diameter of 150 mm or granulating a PZT ceramic containing a lead compound having a large specific gravity, a long drying time is required for thorough drying. This long drying time, however, cannot be secured with a nozzle directed downward.
When the feed liquid contains an inflammable solvent, there is a danger of fire or explosion. When drying is effected in air, substance(s) which is (are) easily oxidized must be prevented from being oxidized. When the waste gas contains an offensive odor, the spreading of the odor must be prevented from the standpoint of environmental pollution control. When an expensive and/or toxic solvent is used, it must be recovered.
Meanwhile, in the conventional method for control of droplet diameter, a large number of nozzle parts were needed; however, it was impossible to always have these parts in stock and selection of the parts of appropriate combination needed skill and know-how. In some cases, the viscosity, concentration, surface tension, etc. of feed liquid change during operation, which necessitated temporary suspension of operation for replacement of nozzle parts.
The present invention has been made in view of the above-mentioned problems of the prior art. An object of the present invention is to provide a nozzle of upward injection type which can atomize even a feed liquid difficult to dry by ordinary methods, a feed liquid of large density or a large droplets into fine droplets and dry them completely by employing a longer residence time in a drying chamber and which can prevent clogging of a spraying nozzle by cooling the spraying nozzle with an atomization gas blown from around the spraying nozzle, as well as a spray dryer using said nozzle of upward injection type.
A further object of the present invention is to provide a spray dryer wherein a high-speed gas for atomization and/or a hot gas for drying are circulated by a closed system for the prevention of fire and explosion, the prevention of solute in feed liquid from being oxidized, the prevention of environmental pollution and the recovery of resource.
A still further object of the present invention is to provide a method for continuously controlling droplet diameters with the feed liquid flow rate and feed liquid pressure being kept constant, without replacing nozzle parts.